In recent years, there have been widespread electronic devices equipped with liquid crystal displays, organic EL displays or the like, and electronic devices equipped with touch panel displays. Glass materials have high surface hardness, and therefore are widely used as materials of cover glasses of the displays of these electronic devices. Such cover glasses are generally subjected to strengthening treatment because glass is a typical brittle material and because decreasing thickness and weight of the electronic devices have created a demand for thinner cover glasses.
Thermal tempering and chemical strengthening are typical examples of strengthening treatment for glass sheets. For thin glass sheets such as a cover glass of a display, the chemical strengthening is generally used. The chemical strengthening is a technique of replacing alkali metal ions contained in the glass surface by monovalent cations having a larger ionic radius so as to form a compressive stress layer in the glass surface. The chemical strengthening is often performed by replacing lithium ions (Li+) by sodium ions (Na+) or by replacing sodium ions by potassium ions (K+).
By performing chemical strengthening treatment on a glass sheet widely commercially available as float sheet glass, a compressive stress layer can be formed in the glass surface. In this case, however, the value of the produced surface compressive stress or the depth of the formed compressive stress layer may be insufficient; for example, the value of the surface compressive stress may be less than 600 MPa or the stress layer depth may be less than 20 μm (see Comparative Example 5 of Patent Literature 1).
In view of such circumstances, there have been reported many inventions relating to a glass composition that can provide a large value of the surface compressive stress or a glass composition that can provide a large depth of the compressive stress layer.
A glass composition suitable for chemical strengthening that is disclosed in Patent Literature 2 contains 64 to 68 mol % SiO2, 8 to 12 mol % Al2O3, 12 to 16 mol % Na2O, 2 to 5 mol % K2O, and 4 to 6 mol % MgO, and in this composition the total content of alkaline earth metal oxides (MgO+CaO+SrO) has been adjusted to 5 to 8 mol % (see claim 1). In order to be suitable for production by the down-draw process, the glass composition described in Patent Literature 2 has a melting temperature of less than 1650° C. and a liquid viscosity of at least 13 kPa·s. In the examples given in Patent Literature 2, the temperature at which the glass has a viscosity of 200 P (200 dPa·s) is 1536° C. or more, and the temperature at which the glass has a viscosity of 35 kP (35000 dPa·s) is 1058° C. or more.
A strengthened glass substrate suitable for use in a touch panel display that is disclosed in Patent Literature 3 contains, in mass %, 45 to 75% SiO2, 1 to 30% Al2O3, 0 to 20% Na2O, and 0 to 20% K2O (see claim 3). However, glass substrates substantially disclosed as examples contain not less than 13 mass % Al2O3; that is, a glass substrate containing less than 13 mass % Al2O3 is not substantially disclosed. Additionally, in the examples of Patent Literature 3, the temperature (T4) at which the glass has a viscosity of 104 dPa·s is 1122° C. or more.
A working temperature and a melting temperature are known as measures of the high-temperature viscosity of glass. In the float process, the working temperature is a temperature at which the glass viscosity is 104 dPa·s, and will hereinafter be referred to as T4. The melting temperature is a temperature at which the glass viscosity is 102 dPa·s, and will hereinafter be referred to as T2.
A chemically-strengthened glass sheet for a display device that is disclosed in Patent Literature 4 is one formed by chemically strengthening a glass sheet that contains 50 to 74 mol % SiO2, 1 to 10 mol % Al2O3, 6 to 14 mol % Na2O, and 3 to 15 mol % K2O and in which the total content of alkali metal oxides (Na2O+K2O) has been adjusted to 12 to 25 mol % and the total content of alkaline earth metal oxides (MgO+CaO) has been adjusted to 7 to 15 mol % (see claim 1). The glass sheet described in Patent Literature 4 is one produced by the float process (see claim 10). In all of the examples, however, T4 is 1083° C. or more. Also, the value of the surface compressive stress is about 600 MPa or less in many of the examples. In examples in which a large value of the surface compressive stress is achieved, ZrO2 is contained.